China has a large amount of water resources, has 7% of the total reserves of global water resources, and ranks sixth in the world. But water in China is also extremely scarce for individual people. For example, water resources per capita are only a quarter of the world average. Also, water resources are unevenly distributed, namely, the south having a plenty of water with poor quality and the north having less water and fewer water resources. With rapid economic development, water shortages have become increasingly prominent. For example, problems related to the water shortages include water waste, utilization rate and gradual deterioration of water pollution. Water resources and water security have become an important factor restricting economic development. Therefore, new unconventional water resources are needed.
Sewage and wastewater may be used as an effective way to effectively alleviate water shortage problems. Compared with other wastewater, wastewater from large urban has some unique advantages with respect to potential for the second water resources. For example, such wastewater is stable, has reliable water supplies, is less effected by seasons and climate, is closed to processing facilities, and has low cost.
After primary and secondary biological degradations, organic contents are referred as secondary effluent organic matter (i.e., EfOM). Composition of EfOM is very complex, and usually includes residual biochemical degradation of organic compounds, natural organic, synthetic refractory organics, microbial products (SMP), disinfection by-products and organic substances that are not yet identifiable. EfOM often makes the secondary effluent to fail to reach the requirement of recycled water; therefore, deep purification is needed.
There are many ways for the deep purification. Depending on the nature of a process, the deep purification may be divided into three types: physical, chemical and biological. Physical methods include adsorption (mainly carbon adsorption) method and membrane filtration method, etc. Chemical methods include chemical coagulation methods, advanced oxidation, ion exchange, etc. Biological methods include biological biofilms such as biofilm membrane filtration methods.
Ion exchange may be used to remove charged organics from wastewater. Secondary effluent of EfOM mostly are negatively charged surface after biochemical systems; therefore, anion exchange resins are used more widely.
Anion exchange resins with magnetic fields have been used recently (e.g., resins made by an Australian company Orica (MIEX) and by Nanjing University (NDMP)). These new techniques not only overcome shortcomings of conventional fixed-bed resins, but also have additional features such as a small size, a fast mass transferring rate, and good performance of sedimentation. This greatly expands applications of resin technology, and therefore anion exchange resins with magnetic fields are used to purify drinking water and sewage. When ion exchange resins purify water, they effectively remove EfOM and other organic matters, greatly reducing generation of disinfection byproducts. Ion exchange resins may also remove ammonia nitrogen, total phosphorus, fluoride, bromide, nitrate, nitrite etc.
However, water treatment processes using ion exchange resins produce a small amount of resin desorption solutions. In general, amounts of desorption solutions vary depending on types of water and range from 0.2-10% of the amount of the water to be processed. In general, drinking water has low-yield desorption solution while wastewater has high-yield desorption solution. Desorption solutions have complex compositions such as high concentrations of organic matters, high salt contents, deep colors, and poor biodegradability characteristics, and therefore their treatment is difficult.
At present, commonly used methods of treating desorption solutions include disposal landfill, incineration concentrated evaporation, and advanced oxidation techniques. These techniques are expensive and therefore limit applications of ion exchange technology in the field of water treatment. Currently, conventional techniques include regeneration of ion exchange resin using a mixture of resins to be regenerated and a regeneration solution by a volume ratio (typically 1:1). Then desorption solution of the regeneration is disposed. Regeneration agents left in the desorption solution has not been fully utilized and also increase the difficulty of treating the desorption solution. At present, a desorption solution using conventional techniques is about 0.2 to 10% of the water to be processed. A large scale of processing of municipal drinking water and wastewater will generate a large amount of desorption solutions. This greatly limits applications of ion exchange technology in the field of water treatment.
A solution to reduce volumes of desorption solutions produced by ion exchange technology and therefore reduce the costs associated with treatment of the desorption solutions is needed.